Distributed feedback lasers, which are mostly used in optical communications, are annoyed by fluctuation of electric field distribution which induces a change of the power ratio Pf/Pr=χ of the forward/backward light. If the front/back power ratio χ varies, a product of a measured forward light power by a constant χ will not give a correct forward light power Pf.
It is considered that the accuracy of monitoring the laser power Pf will be raised by measuring the forward light power Pf directly instead of measuring backward light power Pr by taking account of the fluctuation of the front/back ratio χ. Somebody have proposed contrivances for knowing the forward light power by measuring directly forward light power Pf. The contrivances fully kill the backward light (Pr=0), enhance forward light power, place a slanting photodiode, which reflects most of the laser light slantingly, on a beam line and measure the forward light Pf by the slantingly-placed photodiode.
{circle around (1)} Japanese Patent Laying Open No. 5-342617, “photodiode” proposes a readout apparatus utilizing a laser diode as a light source which places a photodiode in a midway of a light path of the laser diode for monitoring the forward light power Pf. The apparatus reads out data from a rotating photodisc by shooting a bottom of the photodisc with a converged laser beam and observing intensity or polarization of a beam reflected from the bottom of the photodisc. The proposed apparatus places a monitoring photodiode in front of the laser diode instead of the back. FIG. 9 denotes an apparatus proposed by {circle around (1)} Japanese Patent Laying Open No. 5-342617. A laser diode 52 emits light in a forward direction. A grating 53 divides the light into the 0th order beam 63 and a 1st order beam 65. Laser beams 63 and 65 shoot a slanting mirror 54 at 45 degrees. The 0th order beams 63 is reflected and the 1st beam is fully absorbed by the slanting mirror 54. The beam 63 turns the path at 90 degrees by the mirror and goes upward. A lens 59 converges the 0th beam into a parallel beam. Another lens 60 converges the parallel beam into a spot on a point at a bottom of a rotating photodisc 62. The converged beam is reflected by the bottom of the photodisc 62. A disc-reflected beam is reflected by another mirror (not shown) and is sensed by another signal-detection photodiode (not shown).
FIG. 10 is an enlarged oblique view of the mirror 54 proposed by {circle around (1)} Japanese Patent Laying Open No. 05-342617. The surface 57 of the mirror 54 is divided into two concircular regions of an inner full-reflection film 55 and an outer antireflection (zero-reflection) film 56. A triangle-sectioned base 58 keeps the surface at 45 degree inclination. The 0th order beam 63 is fully reflected by the inner full-reflection film 55 and is effectively employed as a read-out beam. The 1st order beam 65 is absorbed by the antireflection film 56. The power of the 1st order beam is measured by a photodiode beneath the antireflection film 56. The power of the forward light is measured by monitoring the 1st order beam 65. No loss accompanies the 0th order beam. The mirror 54 has no mediate reflection part. The grating 53 makes signal light and measuring light from the laser forward light. The signal light and measuring light are spatially divided by the grating 53. {circle around (1)} has no idea of partial-reflection and partial-absorption by utilizing a mediate reflection film. {circle around (1)} is described here since {circle around (1)} is a prior art document of measuring power of forward emitting light. The measurement of the forward emitting light requires such a complicated and sophisticated apparatus consisting of the grating and the concircular full-reflection film and the non-reflection film.
{circle around (2)} Japanese Patent Laying Open No. 08-116127” semiconductor laser” seems to be a reference closest to the present invention. {circle around (2)} tells a problem that exact forward emitting light cannot always be known by monitoring backward light of an LD, because the forward/backward power ratio of an LD fluctuates with temperature changes and power variations. {circle around (2)} alleges that the forward light ratio increases with temperature. {circle around (2)} proposes a laser output controlling apparatus for eliminating the influence of temperature variations upon the output of an LD by monitoring forward emitting light by a photodiode. FIG. 11 demonstrates a structure of the laser diode monitoring apparatus suggested by {circle around (2)}. A stem 70 has a slanting projection 72 with a 45 degree oblique angle. A heatsink 73 is laid upon the stem 70. A laser diode (LD) chip 74 is die-bonded upon the heatsink 73. A photodiode 75 is bonded upon the slanting projection 72 for monitoring forward emitting light of the LD chip 74. Since the slanting projection 72 inclines at 45 degrees, the light sensing surface of the monitoring photodiode 75 also inclines at 45 degrees.
The laser diode (LD) 74 emits both a forward emanating beam Rf and a backward emanating beam Rr. The forward beam Rf shoots the light receiving surface of the photodiode 75 at an angle of 45 degrees. A part of the forward beam Rf is absorbed within the photodiode (PD) 75. The rest of the forward beam Rf is reflected upward by a reflective film on the photodiode and plays a role of working light. The upward reflected light is introduced into an optical fiber for carrying optical signals in optical communications. To minimize loss, the reflective coefficient of the light receiving surface should be small enough.
InP type photodiodes, which have InGaAs light receiving layers, have SiN antireflection films on light receiving surfaces (tops or bottoms). SiN prevents antireflection films from raising reflection rate more than 30%. {circle around (2)} proposes a Si/SiO2 reciprocal layer film of five layers or three layers for enhancing reflective coefficients.
The reflective layer of {circle around (2)} has a five-layered (or three-layered) structure as shown in FIG. 12. {circle around (2)} alleges that 60% reflection rate (a) would be obtained by a three-layered structure of repeating Si and SiO2 layers piled upon an InP window film (Paragraph 30). Here λ is a wavelength of the light emitted by a laser diode and n is a refractive index of the layers composing the reflective layer. Attention should be paid that the refractive indices of SiO2 and Si are different, although the same “n” will be used for denoting refractive indices of film materials in the followings.
(a) Case of 60% reflection; Layers(from bottom to top; bottom in contact with InP)materialslayer thicknessSiO2λ/4nSiλ/4nSiO2λ/4n{circle around (2)} asserted that a five layered structure should be fabricated upon InP window layer of a photodiode.
(b) Case of 90% reflection; Layers(from bottom to top; bottom in contact InP)materialslayer thicknessSiO2λ/4nSiλ/4nSiO2λ/4nSiλ/4nSiO2λ/4n
This is the multilayered film shown in FIG. 12. Since the reflection rate is 90%, 10% of the LD emitting light would go into the photodiode. {circle around (2)} alleges that 10% of the light enables the photodiode to measure the total power of the LD emitting light. {circle around (2)} assertes that thicknesses of SiO2 and Si films should be all λ/4n, namely quarter-wavelength films. However {circle around (2)} does not describe concrete values of a light wavelength λ, thicknesses and refractive indices of the films. Although {circle around (2)} alleged that the reflection rate of the film would be 90%, an exact reflection rate of the proposed film is known to be more than 98% from our precise calculation. There has been no photodiode capable of reflecting LD forward light shooting at a 45 degree incidence angle with a 90% reflection rate prior to the present invention.
{circle around (1)} Japanese Patent Laying Open No. 5-342617 divides laser light into the 0th order beam and the 1st order diffraction beam by a grating, absorbs all the 1st order diffraction beam by the peripheral photodiode positioned at a periphery of the mirror and reflects all the 0th order beam upward by a central mirror. The grating is indispensable, but is too expensive. The concircular mirror, which should have an inner full-reflection mirror and an outer photodiode, would be difficult to produce. The grating and half-division mirror/photodiode would enhance the production cost of {circle around (1)}. Costliness and complexity would be drawbacks of {circle around (1)}.
{circle around (2)} Japanese Patent Laying Open No. 8-116127 proposes a photodiode having a high reflective film composed of quarter-wavelength thick SiO2/Si reciprocal five-layers. {circle around (2)} alleges that the film would have a 90% reflection rate, but it is wrong. An exact reflection rate of the five-layered SiO2/Si reciprocal film is 98%. Too high a reflection rate inhibits a photodiode with a high reflection film from monitoring the forward LD power. A purpose of the present invention is to provide a photodiode capable of absorbing a part of forward LD light and monitoring the forward LD light with a low dark current.